1. Field of the Invention
This invention generally relates to dye-sensitive chemistry and, more particularly, to an ultraviolet (UV) process for the treatment of metal oxide electrodes.
2. Description of the Related Art
Dye-sensitized solar cells (DSCs) offer the potential to provide a practical and economically viable alternative to p-n junction photovoltaic devices. In conventional silicon systems, the semiconductor assumes the roles of both light absorption and charge carrier transport. In contrast, the two functions are effectively separated in DSCs whereby light is absorbed by a sensitizer that is anchored to the surface of a wide band gap oxide semiconductor. In this scenario, charge separation takes place at the interface via photo-induced electron injection from the dye into the conduction band of the solid. Subsequently, carriers are transported in the conduction band of the semiconductor to the charge collector. A large fraction of incident light can be effectively harvested through the integration of photosensitizers exhibiting broad absorption bands in conjunction with oxide films of nanocrystalline morphology. With the appropriate photosensitizer, it is possible to achieve nearly quantitative conversion of incident photons into electric current over wide spectral regions.
In general, conventional DSCs consist of similar architectural components. The first is a transparent anode composed of fluoride-doped tin dioxide (SnO2:F) or similar deposited on a glass plate. On top of the SnO2:F is deposited a thin layer of titanium dioxide (TiO2), which provides a porous structure with extremely high surface area. Conventionally, the TiO2 is nanoparticulate although other morphologies are possible. The plate is then immersed in a solution containing a photosensitive dye dissolved in a nonaqueous solvent. After removing the plate from the photosynthesizer (dye) solution, a thin layer of the dye molecules (monolayer) is effectively anchored to the surface of the TiO2 film. Next, the conductive plate containing the dye-soaked TiO2 film is assembled into a functional DSC device. Although the sequence and manner through which the final steps of fabrication proceed may vary, a metal (platinum) plate (cathode) is brought into contact with the TiO2 film, which is normally accomplished using a thermoplastic spacer. Injection of electrolyte and subsequent sealing of the injection and exit ports completes the cell. Of course, variations in DSC architectures and/or specific methods for their fabrication are possible, although this introduction is limited to the traditional DSC configuration using a solvent-based (I−/I3−) electrolyte.
FIG. 14 is a schematic depicting the operative principles of a DSC device (prior art). Absorption of light by the photosensitizer (S) attached to TiO2 generates an electronically excited state (S*) from which electron injection into the conduction band of TiO2 proceeds. The electrons are subsequently transported through the TiO2 film by diffusion before reaching the anode of the cell (typically an SnO2:F coated glass substrate) and the external circuit (2). The positive charges resulting from the electron transfer (injection) process from the photoexcited sensitizer (S*) are transferred to a liquid electrolyte by reaction of the photosensitizer (S+) with the reduced species of the iodine redox couple (I−) in the electrolyte matrix, which leads to effective regeneration of the photosensitizer ground (S) state (3). Next, the positive charge carrier (I3−) migrates to the cathode to be reduced back to I− by an electron flowing through the external circuit (4). Typically, process (4) requires a catalytic amount of Pt on the cathode surface. Overall, the process control is governed by kinetic competition. For optimized systems, potential loss mechanisms arising from deactivation of photo-excited states (sensitizer), as well as other recombination processes, are largely suppressed through a favorable kinetic balance. Overall, the generation of electrical power from light proceeds in a completely regenerative fashion such that there exists no net change in the chemical composition of the cell.
Although DSC has the potential to provide solar power as a clean, affordable and sustainable technology, many challenges continue to persist. In general, DSCs can potentially provide efficiencies comparable to a variety of thin-film technologies with the added advantage of reduced cost in terms of both materials and processing. Since the advent of DSC technology originally reported by O'Regan and Grätzel in 1991, (B. O'Regan and M. Grätzel, Nature 1991, 353, 737-740) a tremendous effort has been dedicated towards the realization of DSC devices with increasingly higher efficiencies.
Despite the current record efficiencies, most photosensitizers suffer from a severe deficiency in optical absorption at long wavelengths (>700 nm). Furthermore, the choice of photosensitizer is typically limited to either those with broad yet weak absorbance (low molar absorptivity) or others that absorb strongly (high molar absorptivity) over only a narrow wavelength range. In either case, a considerable fraction of the incident sunlight fails to be effectively harvested. Currently, one of the major limitations towards the realization of more highly efficient DSCs exists in the inability to construct a cell with an appropriate photosensitizer that absorbs strongly over broad spectral ranges within a reasonably thin absorbing layer.
FIG. 1 is a diagram summarizing possible modes for binding carboxylic acid group to TiO2 (prior art). In general, two of the most effective anchoring groups for attaching photosensitizer molecules to metal oxide surfaces in dye sensitized solar cells are phosphonic acids [PO(OH)2] followed by carboxylic acids [COOH] which also include their corresponding carboxylate salt, ester, acyl chloride and amide derivatives. In the case of carboxylic acids, efficient binding arises from reactions with hydroxyl groups along the surface although other forms of adsorption are also possible. Binding via both phosphonates and carboxylates is a reversible process although carboxylate binding for ruthenium polypyridyl complexes proceeds with an appreciable equilibrium constant (˜105 M−1). With respect to photosensitizer adsorption on TiO2 via carboxylic acid groups, a number of binding modes are possible which are based upon parameters related to both the structure of the dye and experimental conditions, among other factors. Although it is extremely difficult to propose a universal model for carboxylate binding, the most plausible modes include bidentate chelation and/or bidentate bridging, although multiple possibilities are likely to be simultaneously involved.
FIG. 2 is a mechanistic summary of light-induced hydroxylation along the surface of TiO2 (prior art). The reversible, photo-induced hydroxylation of TiO2 has previously been shown to occur upon exposure to ultraviolet light under ambient conditions, see G. Caputo, C. Nobile, T. Kipp, L. Blasi, V. Grillo, E. Carlino, L. Manna, R. Cingolani, P. D. Cozzoli and A. Athanassiou, J. Phys. Chem. C 2008, 112, 701-714. In this study, thin-film coatings of surfactant-capped TiO2 (anatase) nanorods were oriented laterally along substrates. The evidence for the photo-generation of surface hydroxyl groups on TiO2 was provided through a significant change in water contact angle (CA) following ultraviolet (UV) irradiation (CA ˜20°) relative to the untreated samples (˜110°), which is indicative of a large increase in surface polarity (wettability) for the irradiated TiO2 sample. Furthermore, the reversibility of the phenomena was confirmed by FTIR through the evolution of hydroxyls (upon irradiation) and subsequent disappearance over time during prolonged storage in the dark under ambient conditions. These observations are indeed consistent with the idea that band-gap photo-excitation induces surface defects in the form of oxygen vacancies which allow atmospheric water to compete favorably with O2 for dissociative adsorption, as indicated in FIG. 2.
FIG. 3 is a graph depicting photocurrent density-voltage curves obtained from dye-sensitized solar cells with and without the UV-treated TiO2 electrodes (prior art). The enhanced performance of DSC involving ultraviolet pre-treatment of TiO2 with N719 dye as photosensitizer has been briefly described by F. Hirose, K. Kuribayashi, T. Suzuki, Y. Narita, Y. Kimura and M. Niwano, Electrochemical and Solid-State Letters 2008, 11, A109-A111. Their investigation of dye adsorption using infrared absorption spectroscopy in combination with multiple internal reflection techniques revealed the dissociative adsorption of N719 dye onto hydroxyl groups along the TiO2 surface. Overall, the enhancement was quantified in terms of photocurrent densities that were determined to be 2.27 and 1.77 mA/cm2 for the UV-treated and untreated TiO2 electrodes. Although photocurrent increases were indeed demonstrated following UV irradiation of TiO2, the phenomena is not explicitly correlated with an increase in optical density for the adsorbed (N719) photosensitizer. Furthermore, the study was limited to a single photosensitizer (N719 dye). The UV treatment was done by exposing the surface to UV light with a power of 50 μW/cm2 for 10 min. The photovoltaic performance was measured under visible light intensity of 17 mW/cm2.
It would be advantageous if a UV treatment could make TiO2, and other types of metal oxide electrodes, more sensitive to chemical moiety binding processes.